Products manufactured of welded plastic films primarily include covers, such as prospectus covers, file covers, schoolbook protectors, letter files, or the like. These covers are typically made by welding two planar sheets to one another along one or more weld seams, to define a like number of creases. Available machines for the welding of films for covers and the like do not operate at desirably high speeds, and do not provide automatic separation of the welded plastic films in synchronization with the film welding.
The art seeks a machine for manufacturing products by welding plastic films which operates at high rates and which comprises means for separating the welded objects.
According to the invention, a machine for forming objects by welding plastic sheets is provided which comprises a sheet conveying device. The sheet conveying device comprises a moving carriage carrying film grasping devices. The carriage includes a guide groove receiving a cam roller. The groove extends perpendicular to the direction of travel of the carriage. A cam roller carried by a crank arm mounted on a main drive shaft engages the guide groove, so that as the main drive shaft is rotated, it drives the carriage forward and backward, from an initial position, in which the grasping devices grip the film sheets, to a second position at the end of the carriage travel, where the grasping devices release the film sheets. The sheets are welded in the second position, and are subsequently cut at a separating station to form individual products. The machine according to the invention also comprises a conveyor belt which supports the film sheets between the various stations, and a lifting mechanism which activates the welding press, during the reverse motion of the feed carriage. The lifting mechanism is also driven by the main drive shaft bearing the crank arm driving the carriage.
According to the invention, the plastic film layers in the area of the desired welds are heated to their melting temperature and are subsequently cooled under pressure and without relative motion of them with respect to each other. The welded film sheets may be held by stationary cooling grippers for additional cooling, while the main drive shaft returns the carriage and the grasping devices to the first position, ready to grip the film sheets again to advance them further.
In a preferred embodiment, the machine of the invention comprises additional stationary grippers for holding the film sheets during the reverse motion of the carriage, that is, while the film grasping devices are open. Pneumatic piston/cylinder units activated by solenoid valves activate the grippers and the grasping devices. Precise positioning of the plastic sheets in the processing stations is assured by the phased activation of the moving grasping devices and stationary grippers. The inherent response lag times of the valves and of the piston/cylinder units can be compensated by a per se known electronic time measurement system, and their operation synchronized to the rotational speed of the main drive shaft.
In a particularly preferred embodiment of the invention, a cylindrical cam is placed on the main drive shaft to activate the welding mechanism, which comprises a press having at least one heated anvil. Upon each rotation of the main drive shaft, this cylindrical cam provides precise activation of the welding mechanism. A single-arm lever is preferably inserted between the cylinder cam and the welding mechanism. One end of the lever comprises a cam roller riding on the cylindrical cam; a pivot on the lever supports the welding mechanism, and the fulcrum of the lever is supported by an air bag or the equivalent, which allows control of the pressure exerted by the welding press.
If the rotational speed of the main drive shaft is constant, the time intervals between welds is constant, and the rate of production of the machine is limited by the time taken by the carriage to return to the second position, that is, to bring a subsequent portion of the sheets to the welding station to be welded. That is, if the rate of rotation of the main drive shaft is constant, the travel rate of the carriage in the forward direction, during which the carriage brings a pair of sheets to the welding station to be welded, is equal to that in the reverse direction, during which the sheets are welded and the carriage returns. If film sheets are being processed having mechanical properties which would allow for higher feed velocities (that is, compared to the time required to weld the sheets), which would result in higher production rates, the limitation to equal forward and reverse travel times is very unfavorable.
In a preferred embodiment, machine output is increased by provision of a multiphase ac motor and a control device, driving the main drive shaft transformer which allows reduction of the drive shaft rotational speed during a part of each rotation. As a result, the basic rotational speed of the main drive shaft is set as high as possible, corresponding to the mechanical properties of the films being transported. If the resulting welding time at this rotational speed proves to be insufficient, then the angular velocity of the main drive shaft is reduced when the press is closed and is increased to the set speed just before the press opens again; that is, the welding time is increased.
In this way, the velocity of the main shaft is reduced only while the welding press is closed and the carriage returns to the first position. All other mechanical motions occur at the maximum permissible velocity. More rapid operation and higher output are provided, by comparison with systems in which the welding time is extended by reduction of overall machine drive speed.
Additional details, advantages, and features of the invention are provided in the following description and drawings, to which reference is made expressly for all details not contained in the text.